Heart fibrillation is a high frequency arrhythmia of one or more chambers of the heart. It results in a loss of proper heart pumping action and a corresponding loss of blood circulation.
It is known that fibrillation can be arrested by passing an electric current of sufficient strength through the fibrillating heart. The electric field causes a depolarization of the heart muscle or myocardium. If this depolarization extends to a sufficient amount of the heart tissue, defibrillation can be achieved.
If the fibrillation occurs in a clinical environment, such as a hospital where defibrillation equipment is usually readily available, a pair of disc shaped paddles can be placed upon the chest of the patient. When sufficient electrical energy is applied to the paddles, the required electrical field can be established in the heart. However, this presupposes that fibrillation is detected in time and that the necessary equipment is close at hand.
An alternative solution is to attach a set of electrodes directly to the heart. This would typically be done when access to the heart is provided by open heart surgery or some other surgical procedure. Because the electrodes are attached directly to the heart, the electrical energy required to accomplish defibrillation is much less than the energy required for paddles placed externally on the chest at a substantial distance from the heart.
For people prone to fibrillation symptoms, then it thus becomes practical to implant in the body of a patient a defibrillator that continuously monitors heart activity and automatically and immediately establishes a depolarizing electrical field upon detection of fibrillation.
When the depolarizing electrical field is supplied by an implanted battery, the field must be generated with the expenditure of a minimum amount of electrical energy in order to optimize battery life. Even small energy losses can be important when the energy must be supplied by an implanted battery. It thus becomes difficult to satisfy conflicting demands of physiological factors and electrical energy consumption factors.
From the physiological point of view it is desirable to minimize interference with the operation of the heart. A point contact connected by an extremely flexible wire would be an ideal electrode from the physiological perspective. However, such an arrangement would be less than optimum from the electrical point of view, since it would not provide a uniform electric field in the heart desirable for effective depolarization.
In order to accomplish defibrillation it is necessary to establish a minimum strength depolarizing electrical field throughout a substantial portion of the myocardium. As one would expect, the depolarizing electrical field strength from a small electrode is a maximum at the electrode and decreases as a function of distance from the electrode. From the electrical point of view it is thus desirable to have a large electrode contacting a substantial surface area of the myocardium.
Although the large surface area electrode is ideal from the electrical point of view, it is physiologically unsatisfactory because it imposes a physical restraint upon the heart. The heart must beat continuously about 60 beats per minute and even the slightest interference becomes significant after millions of repetitions. If a heart is of a condition to be in danger of fibrillation to start with, any interference with heart activity becomes even more significant. A compromise is thus generally made between the point contact that is physiologically desirable and the large surface area electrode that is electrically desirable.
U.S. Pat. No. 4,827,932 to Ideker et al. teaches a set of large surface area flat patch electrodes in FIGS. 6a, 6b and 6c which are intended to cover as much of the ventricular surface area of the heart as is possible without inducing large current flows directly between pairs of adjacent electrodes through vascular passages. In the arrangement of FIG. 6b the patch is partially bifurcated to form two projections that may be conformably wrapped about the heart. A laterally connected lead gives rise to a high current density in a base region as current flows past the bifurcation toward the projection farthest from the lead connection point. With such a configuration, the electrical losses which result from the non-uniform current densities can be substantial relative to the available energy from an implanted battery. In addition, the large patch size necessarily imposes a significant restriction upon the expansion and contraction of the heart muscle.
U.S. Pat. No. 4,030,509 to Heilman et al. teaches various arrangements of patch electrodes including a large, contoured electrode for placement at the base of the heart. U.S. Pat. No. 4,291,707 and Des. 273,514 to Heilman et al. teach various arrangements of relatively inflexible flat planar electrodes. Such arrangements do not readily provide substantial contact area without perceptibly impeding the pumping action of the heart.